Very small aperture Ku-band (e.g. 12/14 GHz) satellite terminal (VSAT) networks are currently gaining wide acceptance as an economic alternative to terrestrial communication systems. Advantageously. Ku-band systems enjoy a lower susceptibility to terrestrial radio interference; also the higher carrier frequencies allow Ku-band systems to provide equivalent gain using smaller-diameter dishes. Taken as a group, these factors provide important economic advantageous for large Ku-band VSAT networks. Unfortunately, the higher carrier frequencies also mean increased susceptibility to rain fade.
More particularly, rain fade attenuation is caused principally by scattering and absorption by water droplets. Studies, such as that described in an article by R. K. Crane, entitled "Prediction of Attenuation by Rain", IEEE Transactions on Communications, Volume COM-28, pgs. 1717-1733, Sept. 1980, indicate that the liquid state of the water dominates the attenuation, whereas vapor (fog) and solid state water (frozen ice crystals) associated with clouds do not substantially contribute to attenuation. According to a rain fade model proposed in the above-identified Crane article, attenuation increases principally as a function of frequency and rain rate in accordance with the expression: EQU A=a.times.R.sup.b (dB/km) (1)
where R is the point rain rate, a is a multiplier which is dependent upon frequency, and b is an exponent which is also dependent upon frequency.
A 40 mm/hr rain storm in the path of a transmission between a VSAT earth station and a Ku-band satellite will produce a 10 dB signal fade. Because of climate differences, point rain rates vary greatly with geography. A 40 mm/hr rain rate, for example, occurs, on average, 10 hours per year in Florida. In Maine, on the other hand, such a storm occurs on the average only 10 minutes per year. From a statistical analysis of climate/rain fall conditions, a geographical model of rain rate probability distribution, which provides a basis for anticipating the occurrence of a rain fade, can be derived. The above reference to Crane, for example, describes a global model of a rain fade probability distribution.
Because a rain fade causes a reduction in signal-to-noise ratio (C/N), which must meet a minimum standard for a maximum permissible bit error rate in a VSAT digital communication network, some mechanism is usually provided to adjust one of several variables in the satellite link power budget in order to compensate for the decrease in signal to noise ratio. Among these variables are antenna gain, receiver noise temperature, coding rate and transmit power (EIRP). T. T. Ha, in an article entitled "Digital Satellite Communications", Indianapolis, Ind., Howard Sams and Co., 1987, discusses some of these variables as trade offs for systems with static margin. From a practical standpoint, however, transmit power and coding gain are the only choices for a dynamic or an adaptive fade compensation system. Adaptive coding for rain fade compensation has been proposed for a Ka-band (20/30 GHz) NASA ACTS system, as described in an article by T. Inukai et al entitled "ACTS TDMA Network Control Architecture", Proc. AIAA, 12th International Communications Satellite Systems Conference, pp. 225-239, March 1988. In this compensation system, an earth station using the ACTS satellite would switch to a more robust forward error correction coding scheme during a rain fade. However the complexities of changing coding schemes on-the-fly preclude adaptive coding in less expensive commercial VSAT systems which use linear satellite transponders. For the time being, simple transmit power control is the only economic alternative for commercial VSAT systems.
Static margin is the simplest and most common technique for transmit power control. The static transmit power level includes a margin which provides excess signal-to-noise ratio during clear sky conditions. This additional power protects the link until the rain fade exceeds the margin. As a consequence, unless it is raining, the system user must pay the cost for the extra power margin, so that satellite capacity will be available during rain fades.
To overcome this unused power penalty in a static system, dynamic power control mechanisms, such as those described in articles by M. Seta et al entitled "A Study on the Transmitting Power Control for Earth Stations", Proc. A1AA 12th International Communications Satellite Systems Conference, pp 174-184, March 1988 and S. Egami entitled "Closed-Loop Transmitting Power Control System for K-band Satellite Communications", IEEE Trans. on Aerospace and Electronic Systems, vol. AES-19 pp. 577-583, July 1983, has been proposed. Dynamic power control techniques include independent control, centralized control, pilot reference control and pair control. In an independent control scheme, each station takes care of its own fade without assistance from another station. In the past this has been done by observing the occurrence of a fade and then manually increasing transmit output power in the hope that the impact of the fade will be overcome. Centralized control involves a cooperative central station which measures and broadcasts uplink fade estimates to the remote stations. Pilot control requires each station to estimate downlink fading using a dedicated receiver monitoring a satellite-originate pilot or beacon signal. Pair control involves the use of multiple stations to cooperatively exchange uplink fade observations. Pilot control and pair control rain fade compensation systems are deployed in some satellite systems today. However, since the main reason for employing a VSAT network is its low cost, centralized, pair and pilot control mechanisms are not practical solutions to the rain fade problem. Instead, VSAT systems need an automatic method of independent control for rain fade compensation.
More specifically, a typical VSAT network, a portion of which is shown in FIG. 1, employs a star topology. At the center, or hub, of the star is a master station 10 which sources an outlink satellite channel 12 to the remote stations 20, located at the points of the star. Master station 10 employs a large antenna 14, while each remote station employs a very small aperture dish 24. The communication mechanism between master and remote stations is such that the master 10 transmits on a continuous, powerful outlink frequency (e.g. 14 GHz) on which are modulated individual messages addressed to specific remote stations. The remote stations 20 transmit in burst format over a return link frequency (e.g. 14 GHz) 22. Within the satellite 30 is a shared Ku-band linear satellite transponder with a 12 GHz downlink frequency, which typically has a saturated EIRP of 46 dBW. A minimum 4 dB back-off (namely reduction for peak power) helps prevent intermodulation noise. The outlink modulation scheme uses a fraction of the total available power. Also frequency division access allows the VSAT network to share transponder usage with other systems, including single channel per carrier (SCPC) networks. Because of the cost effective nature of a VSAT network, it can be appreciated that multi or interstation dynamic control mechanisms, such as centralized control, pair control, or the use of an additional pilot/beacon reference, are not practical solutions to the rain fade problem in such a network. As noted above, however, to date independent power control has involved only trial and error adjustment of the transmitted power, an approach which is tenuous at best. These "trial and error" or open loop independent control systems lack sufficient accuracy (greater than ten percent peak error) to permit the master outlink to also act as a pilot beacon, an important operational requirement of VSAT networks.